The present invention relates to methods for reducing and/or eliminating static electric charges on resins, and particularly acrylic resins, by adding certain long-chain alkyl benzene sulfonic acid salts of polysubstituted ammonium compounds, together with antistatic resins containing such sulfonic salts and compositions for such use.
Acrylic polymers find wide commercial use. Their desirable qualities include crystal clarity, good surface hardness, relatively good resistance to scratches, good chemical resistance, and excellent weatherability. They can be readily formed and machined to provide a variety of shapes and are suited for such applications as automotive instrument panel covers, automotive tail lights, paneling, safety glazing, medical applications, sanitary and laboratory wear, aircraft canopies and domes, signage, and the like.
Like many polymers, the acrylics are relatively non-conductive. This can result in problems with static electric buildup and the discharge thereof. There are several aspects to this problem. For example, the sparks which occur in discharging the static charge can act as ignition sources for dusts and vapors with severe safety consequences. Aside from safety considerations, a static electric charge can also attract small particulate material which interferes with the smooth surface appearance of the acrylic and may also decrease its transparency. In the latter case, the need repeatedly to wipe the surface will increase the tendency for the surface to become scratched.
In electrical terms, a material may be considered to be a conductor or an insulator. Conductors have the ability to transmit a stream of electrons and thus carry an electrical current. When insulators are rubbed together, the surface may gain or lose electrons to become negatively or positively charged. Since the material is an insulator, it can not easily dissipate the static charge. It is this static charge which can discharge through spark formation and causes shocks to personnel or fire hazards.
Antistatic agents are materials which are added to plastics to reduce their tendency to acquire electrostatic charge. The antistatic agents are usually either hydrophilic or ionic in nature. When present on the surface of a plastic item, they facilitate the transfer of electrons and thus eliminate the buildup of static electricity.
Antistatic agents have been applied to the plastics in two ways. In the first method which is called an external antistat, the antistatic material, either neat or in solution, is applied to the plastic surface. This method is easily carried out by spraying or dipping. Any type of antistatic material may be used. The major deficiency of this treatment system is that the antistatic agent is easily removed from the plastic surface by rubbing with a cloth or cleaning solution or rinsing with water.
The second type of antistatic agent is called an internal antistat. These materials are added to the plastic before processing. During the processing steps antistatic agent migrates to the surface of the film or molded part. This thin surface layer of antistatic agent imparts antistatic behavior to the plastic surface. If the plastic surface is cleaned and the antistatic layer is removed, more antistatic agent will migrate from the interior of the plastic to the surface and restore the antistatic effect. In this manner an internal antistat produces the antistatic effect for a much longer time.
There are a number of considerations with an internal antistat. The antistatic material must have very good thermal stability to withstand the high processing temperatures required for the plastic. It must not cause color formation or discoloration in the plastic, or adversely affect the physical properties of the plastic. The antistatic agent should also not detract from the appearance of the plastic item. Finally, the antistatic agent must have the proper compatibility with the plastic. If the antistatic agent is totally incompatible with the plastic, the antistat will detract from the appearance or physical properties of the plastic. For example, a clear plastic would become opaque if it contained an incompatible antistatic agent. The ideal antistat would have limited compatibility (or solubility) in the plastic. Only a few materials will have this correct balance of compatibility in any given plastic.
There have been a number of approaches in the prior art for reducing the static charge on acrylic polymers. Sugiura et al U.S. Pat. No. 4,943,380 shows an antistatic composition containing 90-99.9 wt % of polycarbonate or polymethyl methacrylate and 0.1-10 wt % of a heat-resistant phosphonium sulfonate having the formula: A-SO.sub.3 (-) Ri--(+)P(--R.sub.2)(--R.sub.3)--R.sub.4, wherein A is an alkyl group with 1-36 carbon atoms, alkenyl group with 4-24 carbon atoms, phenyl group, phenyl group substituted by alkyl group with 1-18 carbon atoms, naphthyl group or naphthyl group substituted by an alkyl group with 1-18 carbon atoms; R.sub.1, R.sub.2 and R.sub.3 are identical, each being an aliphatic hydrocarbon group with 1-8 carbon atoms or an aromatic hydrocarbon group; and R.sub.4 is a hydrocarbon group with 1-18 carbon atoms.
Govindan U.S. Pat. Nos. 4,973,616 and 5,011,937 show toluene sulfonate salts of 2-alkylimidazolines, e.g., 2-undecyl-4,4,5,5-tetramethylimidazoline. These compounds are said to be useful as internal or external antistatic agents for fibers, e.g., acrylic fibers, and synthetic polymers, e.g., polystyrene.
It is of course desirable that antistatic materials for use with polyacylic materials be heat stable during processing, effective at low levels, inexpensive, and easy to blend with the resin.